Conventional butterfly valves comprise a case 1, a disk 6, and a seat provided for the disk 6 and including an annular elastic seat member 2 fitting in a groove 10 in the inner periphery of the case 1 and having a metal core 21 embedded therein (see FIGS. 8 to 10). Since the seat member 2 has a width coinciding with the thickness of the valve disk 6, it is difficult to hold the seat member 2 in completely intimate contact with the grooved portion 10 of the case 1 or with the metal core 21, thus frequently permitting air to remain between the seat member 2 and the grooved surface 10 or between the seat member 2 and the metal core 21. Consequently when the fluid flowing through the valve is subjected to negative pressure, the remaining air expands, raising the seat member 2 to cause leakage of the fluid while the valve is closed or damage the seat member. Moreover the valve case and metal core must be rendered resistant to corrosion for use with corrosive fluids.
With many of butterfly valves, the outer periphery of the valve disk made of metal is adapted for pressing contact with the elastic seat member to ensure fluid-tightness. The valve seat is formed with holes 43 diametrically opposed to each other for passing the stems therethrough and with circular planar contact faces 22 centered about the holes 43. Each of the contact faces 22 is adapted for intimate sliding contact with a turning face formed on the valve disk and centered about the stem extending therefrom. Since the contact face 22 on the valve seat is circular, it is maintained in contact with the opposed turning face of the valve disk always over a constant area during the turn of the valve disk. Even while the valve disk is opened, therefore, the opposed contact faces are subject to unnecessary frictional contact which produces wear on the elastic valve seat and reduce the smoothness of disk operation.